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Groundwater removal triggered the unusually shallow and deadly
earthquake that hit Lorca, Spain, in 2011, according to a new
study.

Scientists have known for decades that pumping water into the
Earth can
set off small earthquakes. But this is the first time that
removing water has been identified as an earthquake trigger,
researchers said. Both the size and the location of the quake
were influenced by groundwater pumping, the study found.

"The fact that the very tiny stress changes due to normal
processes, such as the extraction of groundwater, could have an
effect on very large systems such as faults, that's very
surprising," said Pablo González, lead study author and a
postdoctoral scholar at the University of Western Ontario in
Canada.

The researchers were also able to precisely calculate the
physical changes that generated the quake. The results will help
seismologists better understand the physics that control when an
earthquake starts and stops — an important step in
predicting when and where a quake will occur, and its size.

"We need observations of this sort to calibrate physical models"
of faults, said Jean-Philippe Avouac, a geologist at Caltech in
Pasadena, Calif., who was not involved in the study. "The
initiation and arrest of [fault] ruptures are something we are
trying to constrain," he told OurAmazingPlanet.

Small quake, devastating effect

The May 5, 2011, earthquake was a relatively moderate magnitude
5.1. Quakes of this size usually don't cause significant damage
in developed countries. A 2006 earthquake of magnitude 4.8 near
Lorca did not cause any deaths. [ Video:
What Does Earthquake 'Magnitude' Mean? ]

But the 2011 quake ruptured only 0.6 miles (1 kilometer) below
the Earth's surface, which meant the earthquake's energy was
concentrated at the surface. Nine people were killed and
dozens were injured, and both unreinforced masonry, like medieval
churches, and modern buildings were damaged.

Thanks to previous research work in Spain, González suspected the
quake's shallow epicenter could be related to groundwater
extraction near Lorca. The groundwater table south of Lorca has
dropped as much as 820 feet (250 meters) since 1960.

"When the tragic event occurred, we asked ourselves if the
earthquake might be related to the subsidence. The earthquake was
very shallow, and moreover, this pattern of subsidence was
bounded by the fault," González told OurAmazingPlanet.

Linking subsidence to fault rupture

Using data from satellite imagery and GPS stations, González and
his colleagues first confirmed the quake occurred on the Alhama
de Murcia fault. Then, they calculated how the crust responded to
removing the weight of the water. Releasing the load increased
the stress on the Alhama de Murcia fault by a few tens of
kilopascals — less than atmospheric pressure — the study found.

During the quake, the fault broke only in areas where removing
groundwater increased stress on the fault. From this correlation,
González inferred that the groundwater removal not only helped
trigger the quake, but also controlled the size of the fault
rupture and the earthquake's magnitude.

However, the amount of energy released by the quake far exceeded
that built up by groundwater extraction. Thus, the earthquake
released both stress caused by groundwater extraction and several
centuries of regional deformation, the study concludes.
Southeastern Spain is near the plate boundary region that
separates the
Eurasia and Africa tectonic plates.

The findings were detailed in the Oct. 21 issue of the journal
Nature Geosciences.